One type of fluid level meter known in the prior art uses a plurality of stacked capacitive level sensors, each having an essentially constant resolution per unit of length. Such sensors can be formed of an outer electrode which is generally continuous, and from an inner electrode can be preferably segmented. Thus, the sensor part of the system is comprised of a plurality of segmented sensor portions stacked from bottom to top, one above another, forming an entire sensor module.
Significant improvements in measuring precision for certain liquids were made possible by apparatus such as that described in U.S. Pat. Nos. 3,301,056 ('056) and by 3,797,311 ('311) to R. L. Blanchard et al. The '311 patent incorporated one or more segmented capacitive sensors ("segments") having a pair of sensing electrodes, which when immersed in a fluid and an a-c voltage is applied to the sensor, provide an electrical output related to the level of the fluid between the electrodes. A reference sensor unit adapted to be completely immersed in the fluid enables control of the driving voltage applied to the sensing capacitor segment in such a manner as to make the output of the sensing capacitor segment independent of both the dielectric constant of the fluid and the driving voltage, and proportional to the ratio of the sensing segment capacitance to the reference segment capacitance.
In order to improve the resolution of such a measuring system, U.S. Pat. No. 3,797,311 suggests use of shorter segments at the extreme ends of the sensor, in combination with one or more longer intermediate inner electrode segments. The segments have generally equal resolution per unit of length, and thus the shorter segments provide greater overall resolution at the extremities (only). The present invention enables calibration of the segments at the time of and in the actual environment of use, and thus presents an accuracy improvement over the prior art, including when such improved resolution techniques are utilized.
As compared with the previous art, the '056 and '311 patents represented great strides in measuring instrument accuracy. Such capacitive fluid level meters have inherently good uniform resolution over the length thereof. Some applications require metering apparatus which has greater resolution over certain portions of the liquid level range. For example, when measuring a very valuable fluid such as liquefied natural gas ("LNG") in the hold of a cargo carrier, it is especially important to have an ability to precisely measure the fluid level at the upper and lower levels in the tank in order to accurately determine the quantity of gas stored or loaded into or off-loaded from the cargo carrier, especially a carrier such as a tanker. On-loading or off-loading cargo transfers, called custody transfers, may involve cargo valuable in many millions of dollars. Such custody transfers may occur 15 or more times a year over many years, with periodic system recalibration taking place only when the ship is in drydock. The system may lose measurement accuracy over time, resulting in highly valuable cargo transfer errors. Aside from the accuracy of instruments per se, has been the need for calibration of the instruments at the time of an in the environment of use.
The prior art provided a useful method and apparatus for estimating and simulating the "empty" and "full" conditions for system adjustment, by providing for the empty, dry or air values. Such system adjustment could be performed by connecting the output of a segment to a summing junction and subtracting from it a signal corresponding to the output of the segment when empty. Provision was made for cancellation of stray pickup effects at the junction by adding electrical signals of appropriate magnitude and phase to cancel the stray pickup. The system segments were initially calibrated for zero and full scale.
Adjustment for a zero setting could be accomplished by such subtracting of the equivalent "empty" capacitance of each sensor segment. To estimate and simulate a full scale condition would normally require experimentally immersing the sensor segment in the proper liquid, a difficult task for very large carriers such as ocean tankers. The level sensor output is effectively the ratio of its capacitance to that of the reference sensor segment, whether or not both sensor segments are empty or fully immersed. Thus full scale calibration is accomplished by disconnecting the empty value compensation of the sensors and setting the indicator to full scale with the sensors empty. The output of each sensor segment signal channel is provided with an adjustment to separately adjust the output of the segment for full scale value.
Effective though the prior art is, it is based on prediction and simulation of the conditions in which the gauging system will be used. Such predictions and simulations are based on typical or average circumstances and are not identical to any specific case. In the present invention calibration is based on actual conditions for the particular sensor, tank, and operating circumstances.
Quite commonly, a plurality of separate tanks is provided in each vessel, requiring multiple calibration efforts, using cables of varying length. Over time, the electronics, sensor and cable characteristics change and thus introduce an error difference which is not accommodated in the prior art measurement method. Calibration during actual loading/unloading measurement, according to the present invention, permits compensation for such errors.
Natural gas is a volatile fluid, some of which is typically lost during long-distance transport. Often carriers are equipped with apparatus to cool the partly empty tanks using small quantities of the LNG stocks. Also, some carrier vessels include locomotion means which use a portion of the LNG as fuel. The tanks are rarely completely emptied upon discharge or completely filled with loading.
It is therefore rarely possible to know accurately what quantity of the fluid is delivered based only on the quantity loaded into the carrier, or based only on tank capacity.
A further improvement in adjustment technology for partly filled tanks is described in U.S. patent application Ser. No. 604,544, now U.S. Pat. No. 4,528,839 for "Transfer Calibration System", which application is assigned to the assignee of the present invention.
The transfer calibrator and the calibration technique proposed in that patent application is for use with a measurement system having a plurality of measurement sensors and sensor segments, a control unit, and a plurality of cables, each cable interconnecting one of the sensor segments to a control unit. In the prior art, the characteristics of each particular sensor segment and its associated cable all have an effect on the measurement made. This situation represents typical shipboard and storage installations. Specifically, the technique described in the application extends the technique described above to the important case of calibration when the tanks contain liquid. This is a usual condition for shore storage tanks and occurs also for shipboard tanks when calibration is required in service, e.g., after electronic repairs. The technique described provides for obtaining and storing initial data on the installed sensor segment and cable combinations which can be used subsequently with the transfer calibrator to simulate the characteristics of the partly filled sensor and the long cables for purposes of calibration as though the tank were empty.
The foregoing "transfer calibration system" is especially useful in level gauging systems when the tanks contain liquid. It restores the calibration capability to the equivalent of the empty tank case.
Thus, the '311 patent provides a valuable means for adjustment of the system when the tanks are empty and dry, and the transfer calibrator described in U.S. patent application Ser. No. 604,544 now U.S. Pat. No. 4,528,839 provides a similar capability when the tanks are partly full. With these adjustments the gauging system is prepared for service use including the first filling. However, the adjustments are based on predictions of the circumstances of use: the density and composition of the cargo vapor, and the capacitances and lengths of the segments at the cargo temperature. Since cargo such as LNG may be maintained at temperatures of -160.degree. C. or so and since the cargo may derive from sources of substantially different composition depending on the country of origin (and indeed on the particular well and the particular year), the estimates on which the adjustments are based are, to a degree, necessarily inaccurate.
There remains a significant need for apparatus and methods for precise calibration of liquid level measurement systems which can be performed rapidly and accurately. Such systems must be rapid, safe, and reliably accurate at the time of custody transfer, or other use. Additionally, positive assurance that the measuring equipment is working within the appropriate performance specifications at the time of measurement is important to buyers, sellers, and international customs and surveying officials around the world. A further need for an historical record of previous calibrations and correction parameters, and the respective dates, exists to give long-term reliability data and measurement confidence. The present invention fills these significant needs.
The present invention calibrates the system in actual use: e.g., with the segments at actual temperature, the segments containing actual vapor, and with the segment capacitances being at their actual values. The residual errors following the warm, dry adjustment are determined and the measurements are corrected accordingly.
An advantage of this invention is that the calibration reference is used in actual circumstances at the time of custody transfer or measurement, rather than on simulations and predictions such as are known in the prior art.
Another advantage of the present invention is the provision of apparatus and methods for automatic calibration of the level gauging system to units of measure which can be traceable to precision national standards which are accepted around the world each time custody transfer occurs.
And another advantage of the present invention derives from the fact that two calibration points are achieved for each segment, providing two values for every segment from which a simple error correctional model may be calculated and stored.
Yet another advantage of this invention is the fact that even a system out of adjustment and producing somewhat erroneous data can be systematically and automatically corrected at the time of the actual measurement.
Still another advantage of this invention is the provision of means for incorporating, storing, and maintaining a data file history of calibrations and corrections.
Another advantage of this invention resides in the fact that previously built systems in existing carrier transports can be readily retrofit with the invention, and related segmented measuring devices of the prior art may be upgraded with minimal cost and effort, all during periodic dry dock or refitting periods.